Electromechanical filter



April 29, 1941. E. -LAKATOS ELECTROMECHANICAL FILTEP 4 Sheets-Sheet 2 Filed Oct. 24. 1939 FIG. 7

FIG. 9

IN VE N 70/? v aLA/mros /$4 Q A T TORNEV Ap 1941- LAKATOS 2,240,306

ELECTROMECHANICAL FILTER Filed Oct. 24, 1938. 4 sheets -sheet 3 F IG.

I 54 56 4 6 26 26 I 28 9 F F/GJZA fi F 63 5 8 26 a F f: 25, i z 25 i 32 36 35 Y U 32 FIG. I28

INVENTOR E. LAKATOS AT TORNEV April 29, 1941. E. LAKATOS 2,240,306

ELECTROMECHAN'ICAL FILTER I Filed Qgt. 24, 19.59 4 Sheets-Sheet 4 Mil/Euro)? By .ELAKATOS 74w. d

ATTORNEY Patented Apr. 29, 1941 warren srnrs Arsur creme. 7

4 Claims.

This invention relates to electromechanical filters and more particularly to improved filters of the type which employs stretched Wires.

In stretched-wire filters of the prior art, as, for example, those of my Patent 2,056,281, issued October 6, 1936, and of Patent 2,086,597, issued July 13, 1937, to R. B. Blackman, energy input and output is efiected by currents passing through tensed Wires positioned in steady'magnetic fields.

While admirable for many uses, diificulties are encountered with such filters, particularly as the frequency range in which they are employed is raised, because of a tendency of the current-carrying wires to vibrate in unwanted modes. In general, too, the impedance levels for which stretched-wire filter-s of the prior art may be conveniently constructed are somewhat lower than the impedance levels of many frequently used communication circuits so that step-up transformers must commonly. be employed at each end of the prior art filters.

Both of the above-described limitations may be avoided and in addition the over-all filter structure may be simplified by employing magneticattraction type electromechanical transducers in place of the el'ectrodynamic arrangements employed in the prior art to "supply power to and withdraw it from stretched-wire'filter structures.

A further difliculty at times encountered with stretched-wire filters of the prior art is that the transverse wires are found to'be sufliciently massive that the flexural stiffness of the wire may introduce undesirable efiects. In such instances, it is shown hereinafter that the transverse Wire may be replaced by a lumped mass and a discrete stiffness, resonant at the same frequency as the trans-verse wire and proportioned to provide a an equivalent mechanical impedance.

It is accordingly an object of this invention to provide simpler and more economical stretchedwire filters, and improved methods of designing and constructing them.

Another object is to eliminate the necessity of employing impedance transformers with stretched-wire filters when they are to be employed in communication circuits of the normally used impedance levels. 7

Another object is to provide an improved type of electro-magnetic electromechanical transducer for use with electromechanical filters.

A further object is to avoid possible difficulties arising from unwanted modes of vibration of the electrodynamic driving mechanisms of stretchedwire filters of the prior art.

Another object is to extend the frequency range within which satisfactory performance maybe obtained from stretched-wire. filters.

Other objects will appear during the course of the following description and in the appended claims. a

The principles underlying the invention may be more readily understood in connection "with the specific embodiments of the invention describ'ed hereinunder and from the accompanying drawings, in which:

Fig.1 shows an improved form oimagnetic at traction electromechanical transducer suitable for use'infilters of this invention; p

Fig.2 is'illustrative of the flux distribution jobtaining in the device of Fig. '1;

.Fig, 3 shows in schematic form an equivalent electrical circuit for the device of Fig.1;

Fig. 'el shows in diagrammatic forma filter of this invention;

Figs. .5 and6 are schematic electric'aleircuit block diagrams'which will beemployed in theex- .planation of the design of structures of this in- .Vention;

Fig. '7 shows the variation of the quantity with flux density B0 of the transducer of Fig. 1;

Fig. 8 shows variation "of the inductance L of 'thetransducer o'iFig. 1 per LGGO' turns o'f the transducer windings with flux density Bo;

Fig. 9 shows the variation cf the index of electrical efiiciencyQe with respect to eddy-current losses for variations in 'fiux density Bo;

Fig-10 shows the variation of'th'e indices of electrical efiiciency Q0 and Q0, with r'espectto copper-losses alone and with respect to over-all losses respectively for variations in flux density "B0;

Fig. 11 shows how additional sections may be added to the filter illustrated in Fig. 4 by merely lengthening the longitudinal wire and adding a transverse wire for each desired additional sec- 7 tion;

'Figs. 12A and 1-23 show filters of this invention employing only a single stretchedw'ire for each filter in addition to the input and output transducers;

Fig. 13 shows a lumped mass supported on a discrete elastance, the combina't'ion'providing an alternative structure for use in place of transverse wires when the'latter cannot 'be'given convenient mechanical proportions and Fig. 14 illustrates the use of the device of Fig. 13 in a filter structure similar to that of Fig. 4.

In more detail, the illustrative filter system diagrammatically represented in Fig. 4 is a bandpass filter and comprises, in addition to the transducers 25 (shown in detail in Fig. 1), a stretched wire 20, the length of which is wave-lengths of the mid-frequency of the band to be transmitted, and a second stretched wire 22, arranged transversely to wire 29. The two wires are firmly attached to each other at their respective center points. Wire 22 is wave-length of the midband frequency long. Cross-hatchings F at each end of both wires indicate points of attachment to a fixed frame support.

The input and output electrical transducers 2", the mechanical construction of which is shown in more detail in Fig. 1, are located A; wave-length of the mid-band frequency from the ends of the wire 20 of Fig. 4. The transducers 25 are of the rocking armature type and the connections be tween the central wire 20 and the armatures 26 may he made by simply mounting the transducers so that the armatures press against the wire. The system is obviously designed to transmit vibratory energy, the direction of which is substantially normal to the common plane of the stretched wires and transducer armatures. The armatures 26 are, as shown in Fig. 1, supported on torsional springs 28, the elastances of the springs and the armature and spring masses are proportioned so that each spring and associated armature is tuned to the mid-hand frequency of the filter. The ends of spring 28 are clamped to supports 21 by blocks 29 and screws 3!. Supports 21 have suitable openings 2! through which the wire 20 may be stretched without physical contact with the supports. On the electrical side of each transducer 25, a series condenser 30, shown in the diagram of Fig. 4, is added in series with the transducer winding 32. The capacity of the condenser is such as to resonate with the inductance of the winding at mid-band. Winding 32 is normally divided into halves, one half being assembled on each of the two vertical portions of core 24. In Fig. 1 the left half of winding 32 has been omitted and parts of the right magnet 34, of the front end of spring 28, of the forward support 21 and of the forward clamping block 29 have been broken away to more clearly show the features of the assembly.

In the explanation of the theory of the structures of this invention the following symbols will be used:

S=area of central wire in square centimeters;

Zx=characteristic impedance per square centimeter of central wire;

=phase shift in radians of a transmission line or in the filters of this invention of a section of stretched wire whose length is wave-length at the mid-band frequency of the filter;

01=the particular value of 0 which occurs at either theupper or lower cut-off frequency of the filter;

1n=the ratio of the characteristic impedance of any particular wire to that of the central 1 wire;

G=force factor, dynes per abampere;

M=eifective mass of transducer armature and its associated supporting torsional spring in grams;

K=effective stiffness of transducer spring in dynes per centimeter;

L=damped inductance of the transducer;

=capacitance of the condenser 30 associated with each transducer.

We will assume that the illustrative system shown in Fig. 4 is symmetrical and that, therefore, we need analyze only one half of it as represented by the network shown in Fig. 5, to the left of the line of symmetry A-A, it being understood that the right half of the network (not shown) is precisely symmetrical with the left half shown in Fig. 5. Sections I and II are proportioned to be infinity type sections having 21;- and 11' radians phase shift, respectively, Zr and Zr are mid-series type image impedances and Zr is mid-shunt. By calculations employing the open and short-circuit impedances in a manner well known in the art of filter design it is found that Using this relationship, the image impedance levels at mid-band. i. e., 0:1r/2, become Z01=Zo2=Zo3=SZK sec 01 (9) The physical elements available with which we are required to simulate the network of Fig. 5 are easily found by inspection of the diagram of Fig. 4. By inspection then the network of Fig. 6 is obtained. Comparing Figs. 5 and 6, it is evident that for equivalence the following approximations are required:

The closest approximations are obtained when M and K and likewise L and C resonate at fo, the mid-band frequency of the filter, (i. e., when 9:1r/2) From the relationship of Equation 10 we get Sf llI tan 0 +tan 0 sec 0 (12) From that of Equation 11 G s ,m3sZK (13) The value of G can be determined from the relationship G Zo.Z03

where Z0 is the image impedance level on the electrical side, and Z03 is the image impedance level on the mechanical side, that is, the value of Z13. at f f It can then be shown that The following table is of interest in showing the values of the various parameters per gram of receiver mass, for a series of filters having 3000-cycle band width between the theoretical cut-offs and various values of mid-band frequencies. In the calculation of L, the electrical impedance desired at each end of the filter was assumed to be 600 ohms.

f 1 ten 91 L525 L(mh) M M 1/M'L Degrees By way of example, taking the band centered at 14,500 cycles and assumin the effective armature mass is .035 gram, the characteristic impedance of the central wire is szK=.035 1575:550 ohms c. g. s.

Assuming that for this member we use piano wire, of density 7.3 and tensed to have a velocity of propagation of 40,000 ems/sec. we get 312,000 ohms c. g. s./sq. cm. (16) Hence S= =1.76 i10 sq. curs. 7 (17) This calls for a wire 0.00583 inch in diameter.

Since the central wire is 3/2 wave-lengths at In,

its length is required to be x =1.63 inches long. 7 (l.

The impedance of the transverse wire is 1111 times thatof the central wire. Since m1=tan 01 (19) m SZ =6D8 55=2030 ohms c. g. s. (20) Assuming that a tungsten wire will be used having v=30,000 and =l9, the characteristic impedance of tungsten wire is 19.0X30,000=570,000 ohms c. g. s./sq. cm. (21) The area required is =3.5 sq. ems. (22) .407 inch. 23)

introduced by the transverse wire at 9650 cycles and 29,000 cycles.

The second method is to replace the trans verse wire by a lumped mass and a discrete stifiness resonating at 14,500 cycles, a convenient physical structure bein very similar tothat of the transducer armatures and springs. An illustrative structure of this type is shown in Fig. 13 and the resulting filter is diagrammatically illustrated in Fig. 14. In Fig. 13 the bar 12 provides a lumped mass, the spring 10 provides a discrete stiffness. The ends of spring 70 are clamped to supporting frame 64 by blocks 66 and screws 68. Bar 12 is normally horizontal and is pressed against longitudinal wire 62 (shown more fully in Fig. 14) in precisely the same manner as the transducer armatures. Openings 65 are provided in supporting frame 64 to give adequate clearance to avoid contact with wire 62.

The effective mass required by this method is calculated from This completes thecalculations required for the design of the illustrative filter.

Any number of additional filter sections, each of 21r radians phase shift may be added by extending the length of the central wire in unitsof 0 per added section and adding an additional transverse wire, or one device of the type illustrated in Fig. 13, per section appropriately spaced along the wire. This process is diagrammatically illustrated in Fig. 11 where a filter having an additional section as compared with the structure of Fig. 4 has been formed by employing a central 0r longitudinal wire 52 the length of which has been increased over the length of wire 20 of Fig. 4, by a distance equal to 26 and a second transverse wire 50 has been added to the two transverse wires 54 and 56 of the structure of Fig. 11 being spaced along longitudinal wire 52, a distance of 20 from each other and from the nearer transducer to each of these wires, respectively, as shown.

For a filter of n sections, each of 211- radians phase shift, the configuration will then consist of a central wire 2120 in length, crossed by n2 i'zdentical transverse wires, spaced at intervals of The simplest filter obtainable, using the element proportions given above is obtained by a length 40. The resulting filter shown diagrammatically in Fig. 12A has 411- radians phase shift and consists simply of longitudinal stretched wire 58 having a length of 49, and a transducer 25 locatedat a distance of 0 from each end of wire 58. A condenser 30 is preferably associated with each transducer winding as described above.

It is possible to derive another type of filter using a single wire and two transducers which can be built up in sections of 1r radians phase shift. This latter type of filter is illustrated diagrammatically in Fig. 123. This type requires a complete recalculation of the results in accordance with Equations 26 to 31, inclusive, given below. This type will be found to have favorable element values for band-pass filters passing relatively narrow frequency bands. The structure as shown in Fig. 123 consists of a central wire 60, 30 long, with a separation of between the driving points, and between each driving point and the nearer end of wire 60. For the general case, employing this structure As mentioned above, the electromechanical transducers 25 of Fig. 1 are of the rocking armature type. The structure and nature of the transducers is shown in Figs. 1 and 2. ,In Fig. 2 only the magnetic circuits of the transducer are shown so that alternating-current and direct-current flux paths may be made more readily apparent. In Fig. 2, large arrows are employed to indicate direct or unidirectional flux paths and small arrows are employed to indicate alternating flux paths. Fine lines are also added to indicate fringing and leakage flux paths. Both the core 24 and the armature 26 are preferably built up by combining a suitable number of appropriately shaped 45 per cent permalloy laminations 0.002 inch thick.

A feature of the transducer design is the use of two permanent magnets 34 to supply the direct-current flux, in the air-gaps in which the ends of the armature 26 are positioned. The arrangement shown has the advantage over the use of a single centrally placed magnet, in that the central portions of the armature 26 carry no direct-current flux. As a result, it is possible, with no increase in armature reluctance to alternating-current flux, to reduce the cross-sectional area of the armature to less than half of that required by the transducers of the prior art equipped with a single magnet.

The effective mass of the armature 25, including that of the associated torsional spring 28, can therefore for the transducers of this invention readily be made .035 gram as assumed in the above-mentioned example.

The calculation of the transducer force factors and inductances, taking leakage into account, is conveniently carried out by the method now commonly employed by those skilled in the art of electromagnetic design.

The method is to calculate the reluctances of the various flux paths indicated in Fig. 2 and described in detail hereinafter and then consider the network made up of a ladder network of inductances, each coil having the reluctance of the appropriate section of the magnetic path and the turns on each coil being the same as the total turns used on the transducer.

Following this idea, the principal alternatingcurrent flux paths in the transducer are determined. The resulting network of inductances is shown in Fig. 3. The force factor Go is that which would be obtained if there were no leakage and all the reluctance were concentrated in the two air-gaps.

. Where N is expressed in units of 1,000 turns, A

is the sectional area in square inches, W is the width of the sides of the vertical portions of core 24 assuming a square cross-section for them, and Z with an appropriate subscript is the length of the path in each instance, then the inductances in henries of the several equivalent coils of the network of Fig. 3 are given by the formulae Where L1 is the inductance of the winding 32 of the transducer 25, L2 is the equivalent combined inductance of the ends of the armature which carry both alternating-current flux and direct-current fiux, L3 is the equivalent combined inductance of the leakage flux between armature 26 and core 24 at both air-gaps, L4 is the equivalent inductance of the longitudinal alternating-current flux path of armature 26, L6 is the equivalent inductance of the vertical portions of core 24, and L1 is the equivalent inductance of the horizontal portion of core 24.

The inductance L5, representing the leakage across the upper ends of core 24, may be calculated approximately by assuming as the corresponding reluctance, three times that of a column of air the cross-sectional area of which is the total surface of one core limb and the length of which is the separation between the faces, or 14. The inductance thus found will be 7 slightly greater than that which would be found by more rigorous methods, but the error is in the majority of cases negligible and in the remaining cases of such a nature as to be substantially harmless.

Hence, we may, for all practical purposes, take L5 as representing the fringing around the air-gap is calculated from an empirical Formula 32 long used in the art. It has been found that the leakage reduces the reluctance of an air-gap by a factor where lp is the perimeter of the vertical portion of the core, A the area of the air-gap and X0 the length of the air-gap. The ratio Zp/A is a minimum for either a circular or a square section. Using a square section whose sides are w, we get the required expression for The force factor Go is given by the formula from the input impedance of the ladder network of Fig. 3.

By way of example, for a transducer of the type illustrated in Figs, 1 and 2 where the core 24 had square vertical members inch on a side, a horizontal member {e inch thick by 2; inch high, and over-all dimensions of inch by inch by s inch, and the armature Was /8 inch by ab inch by 22% inch the equivalentinductances of Fig. 3 were found to be The assumption was made that in L4 and L7 the permeability remains constant at 1,000, independently of the polarization and that those sections that do carry direct-current flux have the same alternating-current permeability for a given polarizing flux in the air-gap.

The procedure for calculating the transducer parameters is then as follows:

Assuming a given air-gap and definite values of flux density in the gap, the corresponding alternating-current permeabilities are found from published data such, for example, as may be found in the Bell System Technical Journal, vol. XV, of January, 1936, at pages 113 to 135, inclusive, particularly ages 124 and 125. This determines the network elements of Fig. 3. Hence, we can calculate Il/I'l Go, G, L and G /L. The last two are shown in curves 40 to 43, inclusive, plotted in Figs. 8 and 7, respectively, for air-gaps of .004 inch and .003 inch. Curves 40 and 42 obtain for the smaller air-gap.

Reverting back to the illustrative filter design, the value of is given as 134x10 The efiective mass of the armature is .035 gm. and hence the required value of r trical Q may be calculated as follows:

(a) Eddy-current losses Knowing the permeability, resistivity and thickness of the sheet material used, we can calculate the dissipation associated with the various inductances in the network of 3. If the Qs, that is the indices of electrical-efficiency as expressed by the ratio of reactance to resistance, of the component coils (L2, L4, L6,' L1, etc.) are high enough, the current distribution is not appreciably different from the dissipationless case. This current distribution is, of course, that which was obtained above when the eifective inductance and force factor were calculated. To obtain the effective resistance, we merely sum up the power losses in the various branches. Proceeding in this manner, we obtain the curves 44 and 45 shown in Fig. 9 for air-gaps of .004 inch and .003 inch, respectively.

(b) Copper losses The curves 42 and 43 of Fig. 8 give the induotance of the transducer winding per 1,000 turns. From the space available and making allowances for Winding emciency, We calculate the resistance of a 1000-turn coil and thus calculate the copper Q or Q0. This is shown in curves 46 and 41 of Fig. 10 for air-gaps of .003 inch and .004 inch, respectively.

The over-all Q is Qe.Qc/(Qe+Qc), shown in curves 48 and 49 of Fig. 10 for air-gaps of .003 inch and .004 inch, respectively.

From these curves it is evident that the copper loss is the controlling factor and hence in the example worked out, it is preferable to work at a flux density of 8,000 lines rather than at 14,900 lines. That the electrical Q is amply large can be seen by noting that for a uniform loss throughout the pass band not to exceed 6 decibels, the minimum electrical Q required is 9.5, while the minimum shown on these curves is approximately 19 and the maximum is 36.

Numerous other applications of the principles of the invention will occur to those skilled in the art. The above-described embodiments are merely illustrative of the application of said principles. The scope of the invention is defined in the following claims.

What is claimed is: s

1. In an electromechanical filter, a magneticattraction electromechanical transducer of the rocking armature type,,said transducer including polarizing means comprising two permanently magnetized members, said members being shaped and positioned adjacent the two ends of th rocking armature, respectively, so that each member will polarize the respective near end only of said armature whereby the direct polarization of said armature is confined to the ends thereof, the mass of the armature may be substantially reduced without increasing the reluctance to alternating flux of said armature and the design of said filter is thereby facilitated.

2. An electromechanical band-pass filter comprising a stretched wire, the length of said wire being three quarter Wave-lengths of the midband frequency of said filter, a first magneticattraction transducer of the rocking armature type disposed to impart vibrating energy to said wire at a point one quarter wave-length of the mid-band frequency from one end of said wire, a second transducer of the same type as said first transducer disposed to absorb vibratory energy from said Wire at a point one quarter wavelength of the mid-band frequency from the opposite end of said wire and an electrical condenser in series with the electrical coil of each of said transducers, the mechanical and electrical portions of said filter being proportioned to pass energy falling withina predetermined band of frequencies and to attenuate other frequencies.

3. An electromechanical filter comprising a longitudinal stretched wire, a plurality of transverse stretched wires, coupled mechanically to said longitudinal wire at their mid-points and spaced apart along the length thereof, a first electromagnetic transducer coupled mechanicall y to said longitudinal wire near one end thereof, a second electromagnetic transducer coupled mechanically to said longitudinal wire near the opposite end thereof, said wires and said transducers being proportioned and tuned with respect to each other and to a. particular frequency whereby the filter is responsive to vibrations in a frequency band centered about said Particular frequency, the transducers being spaced one quarter wave-length of the mid-band frequency of said filter from their respective ends of said wire and the transverse wires being spaced at intervals of one half wave-length of the midband frequency of said filter with respect to each other and the said transducers.

4. An electromechanical filter comprising a longitudinal stretched wire, a first electromagnetic transducer coupled mechanically to said wire near one end thereof, a second electromagnetic transducer coupled mechanically to said wire near the opposite end thereof, and a mechanical vibratory system comprising a lumped mass supported by a discrete elastance, said vibratory system being coupled mechanically to said wire at the mid-point thereof, the said wire, transducers and vibratory system being proportioned and tuned with respect to each other and to a particular frequency whereby the filter is responsive to vibrations in a frequency band centered about said particular frequency, the said wire being six quarter wave-lengths of the midband frequency of said filter, and the transducers being coupled to said wire at one quarter wavelength of the mid-band frequency of said filter from their respective ends of said wire.

EMORY LAKATOS. 

